Within the past few years, a new type of optical fibre that is characterized by a so-called microstructure has received a large interest within the area of optical waveguide research. Optical fibres of this type (which are referred to by several names—e.g., microstructured fibres, photonic crystal fibres, holey fibres, or photonic bandgap fibres) have been described in a number of references, see e.g. Bjarklev, Broeng, and Bjarklev “Photonic crystal fibres”, Kluwer Academic Publishers, 2003. This invention concerns fibres of all types, including those guiding by M-TIR and those fibres guiding by the PBG effect.
Most of the photonic crystal fibres fabricated today are made of silica glass with a number of air-filled holes placed parallel to the fibre axis and extending in the full length of the optical fibre. It should also be noted that few examples of photonic crystal fibres fabricated of other glass types (see e.g., T. M. Monro et al. IEE Electronics Letters, Vol. 36, No. 24, November 2000 for chalcogenide glasses) or polymers (see e.g., A. Argyros et al., Optics Express, Vol. 9, No. 13, December 2001, pp. 813–820) have been reported.
Waveguiding properties of present optical crystal fibre waveguides are limited to properties which can be achieved with silica-based optical waveguides, e.g. wavelength dependent waveguiding properties limited by light attenuation, i.e. light absorption and light scattering.
Thus there is a need for photonic crystal fibres having properties that are not limited to silica.
The idea of applying materials other than silica (or a combination of different material systems) is that it may allow for an even wider range of optical properties to be obtained, e.g., wider (or even new) transmission windows, or different non-linear coefficients—just to mention a few.
It is an object of the present invention to seek to provide improved microstructured photonic crystal waveguides.
It is also an object of this application to describe a class of such new components.
It should be noted that although the present invention is described by analogy to silica-based photonic crystal fibres, it is not limited thereto, as fibres having other base materials also may be used according to the invention.
An interesting possibility concerning the development of new material combinations in relation to photonic crystal fibres is described in the patent application U.S. Pat. No. 5,907,647 entitled “Long-period grating switches and devices using them” by Eggleton et al. This invention describes means to obtain an optical switch by employing a long-period fibre grating for switching light between alternative optical paths. The fundamental elements of the device comprise a variable intensity light source, a length of optical waveguide dimensioned for co-propagating light in two distinguishable modes, and a long-period grating in the waveguide for coupling between the two modes. The waveguide is non-linear so that the effective refractive index is a function of intensity, and as a consequence, the coupling produced by the grating is a function of intensity, and different levels of light intensity can switch between the separate modes of the fibre waveguide. In the description by Eggleton et al., examples of the non-linear function of the waveguide such as glass (possibly doped with telluride or selenide) at sufficiently high intensities or other materials such as semiconductors or organic layered materials are mentioned. An advantage of the invention described in U.S. Pat. No. 5,907,647 is that the signal source and the control source can have the same or different wavelengths, and they can pass through the waveguide in the same or different directions. It is, however, a disadvantage that the devices described by the invention of Eggleton et al. require the inclusion of a long-period waveguide grating, and it is described how coupling between modes of the waveguide may be avoided in the absence of the long-period grating.
Another relevant contribution to the field of photonic crystal fibres (or micro-structured fibres) is described in work by Eggleton and co-workers, see e.g., Optics Express, Vol. 9, No. 13, December 2001, pp. 698–713. In these applications, the micro-structured cladding region is designed to manipulate the propagation of core and leaky cladding modes. The core can incorporate a doped region allowing for the inscription of grating structures, and the air holes can allow for the infusion of active materials yielding novel tuneable hybrid waveguide devices. The resulting hybrid waveguide can be exploited in the design of optical devices such as grating-based filters, tuneable optical filters, tapered fibre devices, and variable optical attenuators. In the tuneable devices described by Eggleton et al., a so-called “grapefruit” micro-structured optical fibre, having six large (about 30–40 micron in diameter) air holes surrounding an inner cladding region of ˜30 microns in diameter. In order to obtain tuneable optical fibre waveguides, Eggleton et al. introduce a polymer (an acrylate monomer mixture) into the six air holes surrounding the core, where after it is UV-cured. The refractive index of the polymer has higher temperature dependence than that of the glass, and since the fundamental mode is not affected by the presence of the air holes, mode guiding in the cladding can be strongly affected by changing the hybrid waveguide temperature by 10–50° C.
It is a problem that the polymer materials that are immersed in the air holes of the fibre devices described by Eggleton have a relatively high viscosity, and that the air holes, consequently, are relatively large, adding limitations to the waveguide forming ability of the mentioned holes. Optical polymers are, furthermore, often showing losses that are significantly higher in the near infrared wavelength range (around 1300–1600 nm) of particular interest to the optical communications industry. Therefore, it would be relevant to use alternative material combinations and means of additional waveguide modification as described by the present invention. It is a further disadvantage of the disclosures by Eggleton that no means for selectively filling the holes are disclosed.
Fedotov et al. “Tuning the Photonic Band Gap of Sub-500-nm-Pitch Holey Fibers in the 930–1030-nm Range”, Laser Physics. 2000. V. 10. N. 5. P. 1086 discloses microstructured optical fibres with filled holes that provide tuning of photonic bandgap properties. Zheltikov et al., Journal of Experimental and Theoretical Physics Vol 93(3) pp. 499–509. September 2001 disclose similar fibres as Federov et al.
It is a disadvantage of the disclosures by Fedotov et al. and Zheltikov et al. that no means for selectively filling the holes are disclosed.
It is an object of the present invention to provide a new class of optical waveguides, in which the optical waveguiding properties may be actively modified in order to obtain tuneability of the optical fibre devices.
It is a further object of the present invention to provide means of introducing active materials, which are soluble in liquids, in the fibre waveguides for applications in components such as light emitters, lasers and amplifiers, e.g. tuneable DFB or DBF fibre lasers.
It is a still further object of the present invention to provide component designs for improved control of the optical fibre components.
It is a still further object of the present invention to provide optical sensor designs, which may allow single-mode as well as multi-mode fibre sensors to be realised.
It is a still further object of the present invention to provide fibre waveguide devices, which are easy to manufacture.